The first U.S. astronauts were a magnificent seven—military test pilots all, three Air Force, three Navy, one Marine—designated to fly Project Mercury capsules into space, then land them back on Earth. When the seven were selected in 1959, no one had ever come close to leaving our planet's atmosphere—and no one knew exactly what might happen if they actually succeeded.

This July 1962 official NASA photograph from Johnson Space Center in Houston, Texas, shows Astronaut Group I in their official gear. Meant to be worn inside a capsule, their silver suits were made mostly of neoprene rubber and aluminized Mylar, built on the same basic principles as airplane-test-pilot suits. Boots, gloves, and helmets were also issued as part of their in-flight safety equipment.

On May 5, 1961, Alan B. Shepard (back, far left) made it into space—and back—just 23 days after Russian cosmonaut Yuri Gagarin became the first human to do so. Less than a year later, John Glenn (front, second from right) became the first American to orbit the Earth. By the last of the six Mercury missions, on May 15-16, 1963, Gordon Cooper (back, far right) circled the Earth 22 times—the equivalent of spending one full day in space.

Group I's careers as astronauts were full of adulation but, by and large, relatively brief. By 1973, Neil Armstrong and others had stood on the moon and, according to the photographer's notes on the back of this image, only two of the original seven were still active in the space program: Shepard and Donald Slayton (front, second from left), both as supervisors to active astronauts. Four others had retired: Glenn, Cooper, Scott Carpenter (front, far right), and Walter Schirra (front, far left).

Gus Grissom (back, middle), the second American in space and the first to make a second trip, died on January 27, 1967, when fire engulfed his Apollo 1 capsule on the launchpad—the first casualty, along with two of his crew, in the U.S. space program.

Project Mercury wasn't just an opening salvo, though. In October 1998, John Glenn—by then a 77-year-old U.S. senator—went up again as part of space shuttle Discovery's mission to study the parallels of the effects of spaceflight and aging.